Make 'geodesic' & 'displaySystem' API properties.

The geodesic-property is used inside of our examples, yet isn't officially
part of the public API.

The displaySystem-property provides a very useful way of configuring the
output of the measurements and should be promoted as well.
Adjust method documentation.
Remove end-of-line whitespace.
Readd dangling space in header.

This allows for easier search and replace operations. The space will
eventually be removed in all files in a seperate commit.
Merge pull request #871 from marcjansen/measure-api

Measure API enhancements